1. Field of the Invention
The present invention relates to a transducer for simultaneously producing compressional and shear waves in a cement sample. More particularly, but not by way of limitation, the present invention relates to a transducer which allows the measurement of the shear wave velocity and compressional wave velocity in a universal cement analyzer cell so as to determine dynamic mechanical properties, i.e., Poisson's ratio and Young's modulus, as well as other properties of a cement sample.
2. Background of the Invention
Ultrasonic cement analyzers (“UCA”) are well known in the art. Compressive strength measurements of a cement sample are best taken using such a device. While the UCA provides a number of advantages over alternative methods for measuring, or estimating, the characteristics of a particular cement sample, a compelling advantage is the ability of the UCA to perform nondestructive measurements. Thus, a single sample may be used to perform a series of measurements, whereas, with alternative methods, a series of samples are required for each measurement to be made. Using a single sample reduces cost, provides more consistent testing, reduces record keeping, saves time, etc.
The UCA was developed to measure the compressive strength of a cement slurry as it sets when subjected to simulated oil field temperatures and pressures. It consists of a high temperature-high pressure autoclave, a heat jacket capable of heating rates up to 5.6.degree. C. (10.degree. F.) per minute, a pair of 400 kHz ultrasonic transducers for measuring the transit time of an acoustic signal transmitted through the slurry, plus associated hydraulic plumbing. The two transducers make transit time measurements through the cement as it sets. A short pulse on a lower transducer propagates through the cement to an upper transducer. Set time and compressive strength are calculated from measured transit time via empirically developed equations. U.S. Pat. Nos. 4,259,868 and 4,567,765 disclose the UCA in detail and are incorporated herein by reference.
It is also known in the art that an acoustic shear wave may be used to determine the setting time of cement. In one method for using a shear wave to determine setting time, a shear wave is generated at a location in a cement slurry. The point in time at which the shear wave propagates through the slurry is indicative of the setting time of the cement. U.S. Pat. No. 5,412,990 issued to D'Angelo et al. discloses such a method for measuring cement thickening time using a shear wave and is hereby incorporated by reference.
In addition, if both the shear wave velocity and compressional wave velocity are known, it is possible to calculate various dynamic mechanical properties of the sample such as Poisson's ratio, Young's modulus, etc. Poisson's ratio is defined as the absolute value of the ratio of transverse strain to the corresponding axial strain resulting from uniformly distributed axial stress below the proportional limit of the material. Young's modulus is the elastic modulus in tension or compression. This information if useful for material development or characterization as well as for quality control purposes. While wave velocities have been employed for the measurement of dynamic mechanical properties in rock specimens, they have not heretofore been used to determine such properties in a cement sample.
An additional disadvantage associated with traditional UCA devices is that, historically, all acoustic cement tests have been run with the cell oriented vertically. With some cement samples, a layer of low strength material forms at the top, which can cause a limitation in the transfer of acoustic energy from the cell/transducer, to the cement at the send end. Further, the low strength material can cause a limitation in the transfer of acoustic energy from the cement to the cell/transducer at the receiving end.
It is thus an object of the present invention to provide a transducer which may be used to simultaneously produce shear and compressional waves in a cement sample.
It is yet a further object of the present invention to provide an ultrasonic cement analyzer which will measure shear wave and compressional wave velocities.
It is still a further object of the present invention to provide a method for measuring the velocities of the shear and compressional waves produced in a sample and to indicate the dynamic mechanical properties of the sample.
It is a further object of the present invention to provide an apparatus and method for overcoming limitations in a transfer of acoustic energy from the cell/transducer to the cement and vise versa due to low strength material adjacent the transducer.